Twilight: What Civil, Nautical and Astronomical Twilight Mean and When Each One Ends

After the Sun slips below the horizon, the sky does not go dark at once. For a while the air above you still carries scattered sunlight, and the light fades in steps. Anyone who has walked home at dusk or waited for the first stars has seen it. That in-between time is called twilight, and its steps have names: civil, nautical and astronomical.

This guide explains what each stage means, how the Sun's position defines it, and why twilight lasts longer in some places than in others. It also gives a few safe habits for watching the sky near sunset and sunrise. It ends with a task in the Rise & Set Dial: set the sky to the end of civil twilight in the evening.

What twilight is

The National Weather Service defines twilight as the period of time before sunrise and after sunset in which the atmosphere is partially illuminated by the Sun. The Sun is already below the horizon, yet its light still reaches you by way of the sky.

To see where twilight starts, you need an exact meaning for sunset. The U.S. Naval Observatory says sunrise and sunset conventionally refer to the times when the upper edge of the Sun's disk is on the horizon. For its calculations, the Observatory places the center of the Sun geometrically 50 arcminutes below a horizontal plane. That number is the Sun's average apparent radius, 16 arcminutes, plus the average atmospheric refraction at the horizon, 34 arcminutes. An arcminute is one sixtieth of a degree, so 50 arcminutes is about 0.83 degree.

Twilight picks up where sunset leaves off. In the evening the Sun keeps sinking, and each stage ends when its center reaches a set depth below the horizon. In the morning the same stages run in reverse before sunrise.

The three stages

The Naval Observatory defines each stage by the position of the Sun's center. A stage begins in the morning, and ends in the evening, when the center of the Sun is geometrically 6, 12 or 18 degrees below the horizon.

NOAA's Global Monitoring Laboratory writes the same limits as solar zenith angles of 96, 102 and 108 degrees. Those equal solar elevations of minus 6, minus 12 and minus 18 degrees.

Civil twilight

Civil twilight ends in the evening when the Sun's center is 6 degrees below the horizon. The Naval Observatory notes that, before it begins in the morning and after it ends in the evening, artificial illumination is normally required to carry on ordinary outdoor activities. The Weather Service describes it as a time when the brightest stars and planets can be seen, the horizon and objects on the ground can be discerned, and in many cases artificial lighting is not needed.

Nautical twilight

Nautical twilight ends when the center is 12 degrees below the horizon. The Naval Observatory explains that the illumination level is such that the horizon is still visible even on a Moonless night, which lets mariners take reliable star sights before sunrise or after sunset. The Weather Service adds that detailed outdoor activities are likely curtailed without artificial illumination.

Astronomical twilight

Astronomical twilight ends when the center is 18 degrees below the horizon. Before it begins in the morning and after it ends in the evening, the Naval Observatory says, scattered light from the Sun is less than that from starlight and other natural sources. The Weather Service says that at this depth the sky illumination is so faint that most casual observers would regard the sky as fully dark, and that the horizon is not discernible. To test the limits of naked-eye observing, the Sun needs to be more than 18 degrees below the horizon, and diffuse objects such as galaxies and nebulae need a totally dark sky.

Why twilight lasts longer in some places

The same 6 degrees does not take the same time everywhere. A Geophysical Institute article from the University of Alaska Fairbanks, written in 1977, explains why. Near the equator the Sun rises and sets rapidly, because its path is near perpendicular to the horizon. There, at equinox, the setting Sun moves from the horizon to 6 degrees below it in only 24 minutes.

At Fairbanks the story is different. The article says the end of civil twilight comes nearly an hour after sunset on equinox dates. The reason for long twilights at high latitude is the slanting path the Sun takes as it passes the horizon. A greater rotation of Earth on its axis is required to bring the Sun 6 degrees below the horizon at the higher latitudes.

The Naval Observatory adds a caution that applies everywhere. The amount of natural light available at any moment during twilight, and how well you can see objects, depends significantly on local atmospheric conditions, cloud cover and the visibility of the horizon. The angles are definitions. They are not light meters.

Watching twilight safely

Twilight begins after the Sun has set, so you do not need to look at the Sun to enjoy it. The Sun is still the hazard around sunset and sunrise, so keep these habits.

• Keep your eyes off the Sun. NASA's eclipse safety page says regular sunglasses, no matter how dark, are not safe for viewing the Sun.
• Wait before you use optics. NASA warns that looking at the Sun through a camera lens, telescope, binoculars or any other optical device can cause serious eye injury, even while wearing eclipse glasses or using a handheld solar viewer. Do not point binoculars or a telescope at the sky while any part of the Sun is still above the horizon.
• Remember that an ordinary sunset is not an eclipse. NASA says a solar eclipse can be viewed directly without eye protection only when the Moon completely obscures the Sun's bright face, during totality. A normal day has no such moment.
• Use the stage times to plan, not to stare. Look up the sunset time for your place, then count forward through the stages.

Common mistakes

• Treating twilight as one thing. It has three stages, and each ends at a different depth of the Sun.
• Thinking night starts at sunset. At sunset the Sun's center is only about 0.83 degree below the horizon. Civil twilight ends at 6 degrees, nautical at 12 and astronomical at 18.
• Expecting the same length everywhere. At equinox the Sun sinks 6 degrees in 24 minutes at the equator, but civil twilight ends nearly an hour after sunset at Fairbanks.
• Assuming it is fully dark after civil twilight. The horizon is still visible during nautical twilight, and the Weather Service says the sky looks fully dark to most casual observers once the Sun is 18 degrees below the horizon.
• Treating the angle as a promise of brightness. The Naval Observatory says visibility also depends on cloud cover and how clear the horizon is.

Try it in the instrument

Open the Rise & Set Dial and set the sky to the end of civil twilight in the evening. The target is a clock time of about 6:30 PM.

The dial pictures a place at middle latitudes near an equinox, with sunset at about 6 PM. At sunset the Sun's center is about 0.83 degree below the horizon, which is the Naval Observatory's 50 arcminutes. Civil twilight ends when the center reaches 6 degrees, so the Sun has to sink about 5.2 degrees more, which is 6 minus 0.83.

How long that takes depends on latitude. The 1977 article gives 24 minutes at the equator and nearly an hour at Fairbanks, both at equinox. Twilight lasts longer at higher latitudes, so a middle-latitude place should fall between those two. The dial uses about half an hour, which puts the end of civil twilight near 6:30 PM.

The later stages follow the same logic, and each needs the Sun to sink another 6 degrees. This request only needs the first. The game accepts a window of clock times around 6:30 PM. The dial is a simplified model, so use an official table for the real times at your place and date.

Questions people ask

Is twilight the same as dusk?

Close, but not identical. Twilight is the whole period before sunrise and after sunset. The Weather Service says civil twilight is also known as civil dawn and civil dusk, so dawn is the morning side and dusk is the evening side.

Can civil twilight last all night?

At high latitudes it can. The Alaska article says that at Fairbanks, from May 15 until July 23, civil twilight never ends because the Sun never falls as low as 6 degrees below the horizon. It adds that even in December civil twilight lasts about an hour there.

Does the Moon change the twilight times?

No. Each stage is defined only by how far the Sun's center is below the horizon. The Naval Observatory's nautical definition mentions a Moonless night only to describe how visible the horizon stays.

Do I need to look at the Sun to watch twilight?

No. Twilight is the time after the Sun has set. While the Sun is still up, keep your eyes off it. NASA says regular sunglasses are not safe for viewing the Sun.

Sources

• Rise, Set, and Twilight Definitions, U.S. Naval Observatory, Astronomical Applications Department. https://aa.usno.navy.mil/faq/RST_defs. Opened November 10, 2026.
• Definitions of Twilight, NOAA National Weather Service, Sioux Falls, SD. https://www.weather.gov/fsd/twilight. Opened November 10, 2026.
• Solar Calculator Glossary, NOAA Global Monitoring Laboratory. https://gml.noaa.gov/grad/solcalc/glossary.html. Opened November 10, 2026.
• Twilight Zone, Geophysical Institute, University of Alaska Fairbanks (T. Neil Davis, October 21, 1977). https://www.gi.alaska.edu/alaska-science-forum/twilight-zone. Opened November 10, 2026.
• Eclipse Viewing Safety, NASA Science. https://science.nasa.gov/eclipses/safety/. Opened November 10, 2026.

We opened each source ourselves on the date shown. The drawings are our own and were checked against the numbers in these sources.

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